Sterile neutrinos in the early Universe

Although the Standard Model of elementary particles successfully describes the Universe up to the smallest known scales, we know that there exists a number of observational phenomena, which do not find explanation in the framework of this theory. Among these problems are Neutrino Oscillations, Dark Matter and the Baryon Asymmetry of the Universe. In this thesis, we are studying the Neutrino Minimal Standard Model (nuMSM), a minimalistic extension of the Standard Model, which can explain all these three phenomena simultaneously, by adding only three right-handed neutrinos to the known three left-handed neutrinos. It is shown that the two heavier sterile neutrinos with masses below the mass of pi-meson, could have been present in such large amounts in the early Universe that they spoil the otherwise excellent agreement between the Standard-Model prediction of light nuclei production during the Big-Bang Nucleosynthesis and the astrophysical observations. In this way, masses of sterile neutrinos are excluded from below, which reduces the potentially interesting parameter space for future accelerator searches. Another effect of sterile neutrinos in the early Universe that is studied is the production of large-scale magnetic fields due to the so-called Chiral Magnetic Effect.